Operation of a combustion zone such as that found in a circulating fluidized bed combustor requires that the inventory of solids in the combustion zone be maintained at a specified level. The particle size distribution of the bed material in the combustion zone is also critical for proper operation. Because the feed, typically a solid hydrocarbon material as the fuel and an alkaline material as an adsorbent for sulfur capture, contains non-combustible ash components, there is a constant need to withdraw ash from the combustion zone. Finer ash particles are typically elutriated from the combustion zone and lost as fly ash. The elutriation rate for fly ash is generally difficult to adjust, and tends to remain invariant for a given combustor design. Consequently, the quantity of bed inventory typically is maintained by adjusting the rate at which additional ash is withdrawn from the combustion zone. This material is usually designated as bottom ash. The particle size distribution of the bed material will generally be a consequence of the uncontrolled fly ash elutriation rate, and the intrinsic agglomeration and attrition rates of the feedstocks. It can be influenced indirectly by the feed size distributions and the bottom ash withdraw rate.
The conventional operating practice of controlling the inventory of solids through control of the bottom ash withdraw rate, coupled with a lack of direct control on the particle size distribution of the bed inventory, results in several problems. Due to the intense mixing inherent in the design of a circulating fluidized bed combustor, the bottom ash withdrawn from the combustion zone is typically well mixed; i.e. its composition is similar to that of the average bed inventory. This material will necessarily include some fuel and adsorbent particles, as well as inert ash particles. While it is desirable to remove the inert ash particles from the combustion zone, it is inevitable that some fuel and adsorbent particles will also be rejected. Consequently, the bottom ash can contain a significant amount of unreacted feedstock, i.e. fuel and adsorbent materials. Recent investigations have shown further that the coarser bottom ash particles are relatively rich in unreacted fuel. Adsorbent losses can also increase when large limestone particles are used for sulfur capture. As is known in the art, the sulfation of larger limestone particles tends to cause plugging of the exterior particle pores, leaving an interior region which is unreacted. Hence the current methods of controlling the solids inventory results in a loss of unreacted fuel and adsorbent, which increases the operating cost for feeds, as well as costs for ash disposal. In addition, these techniques do not allow direct control of the particle size distribution of the bed inventory.
A further problem in the operation of a circulating fluidized bed combustor is the need to selectively remove large particles from the solids inventory in the combustion zone. It is generally known in the art that excessively large particles in the bed inventory can be detrimental to operation, even in small concentrations. These particles contribute disproportionately to erosion in the lower portions of the combustion zone. In addition, their inherently reduced heat transfer characteristics can allow them to act as nucleation sites for agglomeration. Once the agglomeration process begins, its rate tends to increase exponentially with particle size. In severe cases, agglomeration can cause the process to shut down completely. Consequently, there is a need to be able to remove undesirable large particles that are fed or which may form in the combustion zone, even if they are present in small quantities. Because the current methods for bottom ash withdraw are generally not size specific, these procedures can only remove the harmful size fractions by gross purging of the solids inventory. This practice will usually require an excessively high bottom ash withdraw rate, which will deplete the bed inventory and adversely affect the process operation. Hence, current methods do not permit excessively large particles to be practically purged from the combustion zone.
Thus there is a need to be able to independently control the quantity of the solids inventory and the particle size distribution of the solids inventory to permit recovery of unreacted feedstocks from the bottom ash and selectively remove detrimental size fractions from the combustion zone. The use of this invention will meet this need, thereby providing several operational benefits for the operation of a combustion zone such as that found in a circulating fluidized bed combustor. It will decrease the amount of feed materials which are lost in the bottom ash, thus saving both feedstock costs and ash disposal. It will also permit control of the particle size distribution of the solids inventory independent of the quantity of bed inventory. Consequently, excessively large particles can be purged from the bed inventory without adverse operation effects. This feature will reduce erosion in the combustion zone, and provide a method for controlling agglomeration. In addition, it can extend the operability range of the technology to use certain types of fuels and adsorbents which were previously too difficult to utilize in these combustors because their ash content is either too low, resulting in an insufficient quantity of bed material; or their ash is too friable, resulting in the too fine a particle size distribution. With such feedstocks, it is difficult to establish or maintain the solids inventory without direct, independent control of the particle size distribution of the bed material.
The need to control both the quantity and particle size distribution of the solids inventory in a circulating fluidized bed combustor has lead to several techniques which tend to cause these two parameters to be coupled, as discussed above. The individual problems, such as feedstock losses, erosion control, and prevention of agglomeration are also addressed by several methods. Feedstock losses in the fly ash have been addressed in the prior art, including the use of fly ash re-injection taught in U.S. Pat. No. 4,981,111 by Bennett et al. Losses in the bottom ash are typically minimized by design and operation considerations. For example, the particle size distribution for the fuel and adsorbent is usually specified to reduce the amount of unreacted material rejected from the combustion zone. Size specifications for fuel and adsorbent can help to reduce losses in the bottom ash. However, other considerations, such as pressure drop through the combustor, heat transfer requirements, and combustor stability usually are also considered in the specification of feed size distribution. As a result, the feed size can not often be optimized to minimize feed losses in the bottom ash. Indeed, some losses through the bottom ash are inevitable due to the inherent mixing of the solid phase in a circulating fluidized bed combustor.
In some installations, bottom ash is classified, i.e. separated by size fraction, prior to final discharge from the combustion zone in an attempt to strip the finer particle sizes from the bottom ash stream. Usually these classifiers strip the finer particles by contacting them countercurrently with an air stream. See for example U.S. Pat. No. 4,829,912 by Alliston et al. While this technique reduces the losses associated with the finer particles in the bottom ash, it does not recover the fuel or adsorbent lost in larger particles. Depending on the nature of the feed and operating conditions, the feed losses in the larger particles in the bottom ash can be comparable or greater than the losses in the finer particles in the bottom ash.
There have been several investigations into recovering unreacted adsorbent from bottom ash. These typically involve a chemical treatment of the ash, such as contacting with alkali or hydration. The chemical action is used to increase the availability of adsorbent inside the bed particles to the gas phase reactants.
Erosion problems are usually handled by placing sacrificial or wear-resistant materials in erosion-sensitive areas of the combustion zone. Typical examples include spray coatings or refractory applied to heat transfer tubes, or the use of high grade alloys to construct the heat transfer tubes for the lower regions of the combustion zone.